<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function Person() {
        // this = {}
        // this.__proto__=Person.prototype
        this.username = 'jack';
        this.say = function () {
          console.log('hello' + this.username);
        };
        // return this;
      }
      Person.prototype.age = 20;
      const jack = new Person();
      console.log(jack.__proto__);
      console.log(jack.age); //20
    </script>
  </body>
</html>
